The present invention relates to overload protection for an electrical load such as a motor. In particular, the present invention relates to a contactor control circuit which opens the contactor in response to the detection of an overload.
Electric motors are one type of electrical load which can be started and stopped using a contactor. The contactor includes a contact associated with each phase conductor connected to the motor. Thus, for a three-phase motor, the contactor would include three contacts which are opened and closed in unison. The mechanical motion required to open and close the contacts is provided by a solenoid including a coil. The coil is controlled by a basic circuit which includes a normally closed stop button, a normally open start button, and an overload switch. Upon depression of the start button, the solenoid coil is energized and maintained energized to close the contacts of the contactor. A holding circuit maintains the coil energized when the button is released. The present invention is directed to the control of the overload switch.
It is known to control the overload switch of a contactor using overload relays which include bimetal elements which are designed to generally model the heating and cooling characteristics of the associated motor. The bimetal element is normally heated by the current being applied to the motor. Accordingly, when the current applied to the motor exceeds a certain percentage of the full load current applied to the motor, the bimetal element deforms and opens the overload switch, thus directly disconnecting power to the motor or disconnecting power to a contactor coil which controls power to the motor.
Another known type of overload relay includes a microprocessor based controller which monitors the current flowing through the phase conductors attached to the motor. Based upon samples of the values of the current being applied to the motor, the microprocessor estimates the temperature of the motor and controls the overload contactor when the estimated temperature of the motor exceeds a predetermined limit. Other microprocessor based systems may also include temperature sensors mounted on the motor. The microprocessor compares the sensed temperatures with predetermined limits and opens the overload switch when predetermined temperature limits are exceeded.
The above described systems, while providing satisfactory overload protection, are subject to a number of problems. One problem with the bimetallic based overload relay is the inability to accurately and effectively tailor the characteristics of the bimetallic actuator to the characteristics of the motor being protected. Microprocessor based overload relays are subject to problems caused by the very noisy environments which are typical around loads such as electric motors. Accordingly, with both the bimetallic based overload relays and the microprocessor based overload relays, there are problems with the control schemes which tend to reduce either the overload protection reliability or suitability for a particular load. It is feasible to solve the problems with bimetallic and microprocessor based overload relays; however, these solutions may be relatively expensive and unworkable for high volume products.
Accordingly, it would be advantageous to provide an overload relay which incorporates the beneficial features of the bimetallic and microprocessor based overload relays while reducing the effects of the disadvantages of these relays.